[PATCH v1 net-next 04/10] ipv4: fib: Drop RTNL annotation for net->ipv4.fib_table_hash[].

[Kuniyuki Iwashima <kuniyu@google.com>]

fib_newrule() will drop RTNL except for the first IPv4 rule.

net->ipv4.fib_table_hash[] will be read with no protection,
but this is fine because fib_table is not destroyed until
netns dismantle except for the merged main/local table.

fib_unmerge() will continue to be called under RTNL, so other
readers (fib_flush() and fib_info_notify_update()) just have
to care about the concurrent hlist_add().

IPv6 and IPMR/IP6MR also take this strategy and use RCU helpers
to avoid data race against concurrent hlist_add().

Let's not use lockdep_rtnl_is_held() and rcu_dereference_rtnl()
for net->ipv4.fib_table_hash[].

Note that commit a7e53531234d ("fib_trie: Make fib_table rcu
safe") started to use the _safe version in fib_flush(), but it
is not needed thanks to RTNL.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 include/net/ip_fib.h    |  3 ++-
 net/ipv4/fib_frontend.c | 23 +++++++++++++----------
 net/ipv4/fib_trie.c     |  3 +--
 3 files changed, 16 insertions(+), 13 deletions(-)

diff --git a/include/net/ip_fib.h b/include/net/ip_fib.h
index c63a3c4967ae..0a35355fb0f3 100644
--- a/include/net/ip_fib.h
+++ b/include/net/ip_fib.h
@@ -302,7 +302,8 @@ static inline struct fib_table *fib_get_table(struct net *net, u32 id)
 		&net->ipv4.fib_table_hash[TABLE_LOCAL_INDEX] :
 		&net->ipv4.fib_table_hash[TABLE_MAIN_INDEX];
 
-	tb_hlist = rcu_dereference_rtnl(hlist_first_rcu(ptr));
+	/* Only fib4_rules_init() adds fib_table. */
+	tb_hlist = rcu_dereference_protected(hlist_first_rcu(ptr), true);
 
 	return hlist_entry(tb_hlist, struct fib_table, tb_hlist);
 }
diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index 336d70649eb9..54eb72695093 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -126,24 +126,28 @@ struct fib_table *fib_new_table(struct net *net, u32 id)
 }
 EXPORT_SYMBOL_GPL(fib_new_table);
 
-/* caller must hold either rtnl or rcu read lock */
 struct fib_table *fib_get_table(struct net *net, u32 id)
 {
-	struct fib_table *tb;
+	struct fib_table *tb = NULL;
 	struct hlist_head *head;
 	unsigned int h;
 
 	if (id == 0)
 		id = RT_TABLE_MAIN;
 	h = id & (FIB_TABLE_HASHSZ - 1);
-
 	head = &net->ipv4.fib_table_hash[h];
-	hlist_for_each_entry_rcu(tb, head, tb_hlist,
-				 lockdep_rtnl_is_held()) {
+
+	/* fib_table is not destroyed until ip_fib_net_exit()
+	 * except for the merged main/local table.
+	 * fib_unmerge() is called under RTNL, so other readers
+	 * under RTNL (e.g. fib_flush(), fib_info_notify_update())
+	 * can safely traverse the list with rcu_dereference_raw().
+	 */
+	hlist_for_each_entry_rcu(tb, head, tb_hlist, true)
 		if (tb->tb_id == id)
-			return tb;
-	}
-	return NULL;
+			break;
+
+	return tb;
 }
 #endif /* CONFIG_IP_MULTIPLE_TABLES */
 
@@ -206,10 +210,9 @@ void fib_flush(struct net *net)
 
 	for (h = 0; h < FIB_TABLE_HASHSZ; h++) {
 		struct hlist_head *head = &net->ipv4.fib_table_hash[h];
-		struct hlist_node *tmp;
 		struct fib_table *tb;
 
-		hlist_for_each_entry_safe(tb, tmp, head, tb_hlist)
+		hlist_for_each_entry_rcu(tb, head, tb_hlist, true)
 			flushed += fib_table_flush(net, tb, false);
 	}
 
diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index e11dc86ceda0..d1d342d7148e 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -2137,8 +2137,7 @@ void fib_info_notify_update(struct net *net, struct nl_info *info)
 		struct hlist_head *head = &net->ipv4.fib_table_hash[h];
 		struct fib_table *tb;
 
-		hlist_for_each_entry_rcu(tb, head, tb_hlist,
-					 lockdep_rtnl_is_held())
+		hlist_for_each_entry_rcu(tb, head, tb_hlist, true)
 			__fib_info_notify_update(net, tb, info);
 	}
 }
-- 
2.55.0.rc0.799.gd6f94ed593-goog